Acoustic microscopes and other similar acoustic inspection systems are utilized to determine physical characteristics of objects that cannot be ascertained by more conventional optical examination techniques. Early examples of scanning laser acoustic microscopes are presented in U.S. Pat. Nos. 3,585,848 and 3,790,281; a more effective scanning laser acoustic microscope is disclosed in U.S. Pat. No. 4,012,951. These acoustic microscopes make it possible to visualize localized variations in elastic properties, particularly the density and elastic modulus (or its reciprocal, compressibility), for a wide variety of different objects.
Defects and other variations in a sample object, such as flaws in laminations or variations in density or porosity, act as attenuators or scatterers for the acoustic beam in an acoustic microscope. That is, localized structural features in an object subjected to examination in an acoustic microscope differentially attenuate, absorb, or reflect acoustic energy to produce patterns of light and dark features (grey scale) within an acoustic image produced as the output of the microscope. Quantification of acoustic properties within the object under examination, particularly if effected on a localized regional basis, can be of substantial importance in providing a basis for identifying defects in the object. The amplitude level of the image signal used to develop the acoustic image is generally responsive to the localized acoustic signal level, so that measurement of image signal level may be taken as a first step toward quantification of the image information.
However, there are a number of substantial problems associated with measurement of the amplitude level of the acoustic image signal which make that signal level unreliable as a direct source of quantified data pertaining to the acoustical properties of an object under examination. Thus, in the present state of the art the dynamic ranges of the amplifiers, detectors, and other circuits utilized to develop the acoustic image signal are often insufficient to allow for accurate reproduction, on a quantified basis, of the localized attenuation characteristics of an object subject to inspection in an acoustic microscope. This is also true of the available cathode ray tube image display devices, which have a rather limited dynamic range.
Another problem is that the detection, demodulation, and other signal processing circuits incorporated in an acoustic microscope are not usually linear over the entire dynamic range of operation for the microscope. This non-linearity difficulty applies also to the cathode ray tube displays usually used to produce the acoustic image output. These non-linearities effectively preclude a valid quantitative comparison between high and low image signal levels. To some extent, this may be rectified by calibration of the circuits and the display, based on samples having known acoustic properties. However, even such calibration can prove ineffective due to the fact that the gain characteristics of amplifiers and the impedance values of other electronic components may drift over extended periods of time. Such drift also occurs with temperature changes, especially during the warm-up cycle for the acoustic imaging system. Thus, an ordinary calibration technique applied to the demodulation, detection, signal processing, and display equipment may require repetition for each measurement, a time-consuming and inefficient procedure.
Another inhibition with respect to quantitative comparison of data from acoustic imaging systems results from the fact that the image signal contains data for a complete field of view defined by the scanned area of the object under test. This makes it difficult to compare one area of a given sample object to another area of the same object, or even to obtain quantitative information specific to a limited portion of an object being subjected to acoustic inspection. Further, there is no effective technique for useful quantitative comparison of limited portions of two different objects.
In an acoustic imaging system that employs optical (light beam) scanning, any non-uniformity of the scanning illumination intensity can cause errors in interpretation and quantification of the acoustic image. Such non-uniformity may result from non-uniform deflection in the scanning system, from stage misalignment relative to the photodetector, from non-uniform photodetector response characteristics, from variations in a reflective surface incorporated in the scanner, or from a number of other causes. Elimination of such scanner-induced errors may be essential to effective operation of a scanning acoustic microscope, particularly when employed for quantitative as well as qualitative measurements.
Throughout this application the terms "acoustic imaging system" and "acoustic microscope" are used interchangeably; an acoustic microscope constitutes any acoustic imaging system affording any degree of magnification. Further, both terms are intended to encompass combined acoustic-optical imaging systems (microscopes) that provide both acoustic images and optical images of an object under examination, as in the system disclosed in U.S. Pat. No. 4,012,951.